The present invention relates generally to high bandwidth cathode ray tubes, and more particularly to an improved slow wave deflection structure for use in traveling wave deflection systems.
Traveling wave devices are well known in the art. Traveling wave devices address the problem that, since it takes an electron beam a finite amount of time to travel from a source electrode to a destination in an electron tube, that beam cannot be accurately deflected to reflect variations in a modulating source which occur within that finite time. The solution provided by traveling wave devices is that wave deflection forces be made to "travel" along with electrons on their traverse between the emission source and the destination.
In traveling wave cathode ray tubes, a slow wave delay device is necessary for delaying deflection signals such that a traveling deflecting field which approximates the speed of the electrons to be deflected is produced. The slow wave device generally takes the form of a deflection structure which causes the signal to meander along a circuitous path, and such deflection structures have generally taken the form of helices or serpentines. Several variations of slow wave deflection structures are known in the prior art. An example is the variation taught by U.S. Pat. No. 3,504,222, issued to Fukushima. The Fukushima invention improves the dispersion characteristics of traveling wave tubes which employ the teachings of that invention.
A problem of particular concern in the operation of traveling wave cathode ray tubes is that caused by fields which are not confined to the helical or serpentine transmission line, but are coupled, instead, between turns of the helix or serpentine or are transmitted by higher velocity modes in space between the helix and ground planes. These fast fields deflect the electron beam at a point on the beam where no deflection should occur, and cause an erroneous signal to appear where there should be none. This particular erroneous signal is called a "precursor" artifact. The problem which causes the precursor artifact is more serious and detrimental in high bandwidth cathode ray tubes, because the high frequency components of these signals are preferentially coupled along the structure of the slow wave device.
It is known in the prior art that precursor signals can be eliminated by means of specialized helical structures or by interposing grounded metal fins between the adjacent bars of serpentine slow wave structures. However, the simple imposition of grounded metal fins does not prevent the appearance of resonances caused by characteristic impedance discontinuities at the "loops" of the serpentine, nor does it provide a discontinuity-free transmission line necessary for propagation of high fidelity and high bandwidth signals.
No prior art slow wave device, to the inventors' knowledge, has successfully provided an effective means for producing a delayed deflection signal which both eliminates fast forward coupling and is free from discontinuities. All prior art means for producing slow wave deflection signals using serpentine deflection structures have either not eliminated fast forward coupling and/or have not provided a discontinuity-free transmission line.